Weird network/wifi issue with Desire ZSupport

So here is the problem I am having with my Desire Z. It has already been replaced once so it's not a faulty phone.When I get home it automatically connects to my wifi. It works fine for quite a while but then at some point my texts stop working and the phone doesn't ring when people call. If I turn off the wifi on the phone at this point it will just show the 4 bars at the top but no 3g or HSPA logo. If I reset the phone it will work correctly again.

I am now going to test for a day or two by not having the wifi on at all and see what happens.

So from this I gather that the phone is losing its connection to the network at some point when wifi is on. Or perhaps (although it seems unlikely) the wifi is causing the phone to disconnect or malfunction. As I said the phone has been replaced already for this issue as we thought it was the phone itself, but the replacement does the same thing. I really cant figure this one out at all.